Providing a voice session in an LTE system may be done by VoLTE, i.e. voice over LTE, relying on IMS, IP Multimedia Subsystems, using Packet Switched, PS, technology.
Since LTE systems may, at least initially, have limited coverage, a VoLTE session may have to rely on Single Radio Voice Call Continuity, SRVCC, i.e. continuation of the voice session in an available 2G/3G system using Circuit Switched, CS, services when a user, a UE, loses LTE coverage. A VoLTE session is initiated by a UE based on an indication sent from the Network or system, indicating if VoLTE is available or not for the UE.
In certain situation in which VoLTE and/or SRVCC is not available, LTE users may have to rely on so called Circuit Switched Fallback, CSFB for voice service, i.e. the voice session is initiated and carried out using an available 2G/3G system.
SRVCC handover from an LTE system to a 2G/3G system provides functionality for session continuity: when a UE starts to lose LTE coverage during a voice session, it needs to make handover to an available cell in a 2G/3G system. The SRVCC function ensures that the IMS system can route the voice session via a mobile Switching Centre, an MSC, to the 2G/3G system.
Naturally, the LTE system's support for SRVCCC handover to 2G and/or 3G systems needs to match the UE's capability for operation in 2G and/or 3G systems, or SRVCC handover may fail, resulting in the voice session being lost.
In an LTE system, it is the Mobility Management Entity, the MME, which indicates to a UE if a voice session using VoLTE should be initiated, or if the US instead should use CS Fallback for a voice session.